linux-kernel - Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for TDP MMU

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Message-ID: <20240524082006.GG212599@ls.amr.corp.intel.com>
Date: Fri, 24 May 2024 01:20:06 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>,
	"sagis@...gle.com" <sagis@...gle.com>,
	"dmatlack@...gle.com" <dmatlack@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>,
	"Aktas, Erdem" <erdemaktas@...gle.com>
Subject: Re: [PATCH 10/16] KVM: x86/tdp_mmu: Support TDX private mapping for
 TDP MMU

On Thu, May 23, 2024 at 11:14:07PM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> On Tue, 2024-05-14 at 17:59 -0700, Rick Edgecombe wrote:
> > +static void handle_removed_private_spte(struct kvm *kvm, gfn_t gfn,
> > +                                       u64 old_spte, u64 new_spte,
> > +                                       int level)
> > +{
> > +       bool was_present = is_shadow_present_pte(old_spte);
> > +       bool was_leaf = was_present && is_last_spte(old_spte, level);
> > +       kvm_pfn_t old_pfn = spte_to_pfn(old_spte);
> > +       int ret;
> > +
> > +       /*
> > +        * Allow only leaf page to be zapped. Reclaim non-leaf page tables
> > page
> > +        * at destroying VM.
> > +        */
> > +       if (!was_leaf)
> > +               return;
> > +
> > +       /* Zapping leaf spte is allowed only when write lock is held. */
> > +       lockdep_assert_held_write(&kvm->mmu_lock);
> > +       ret = static_call(kvm_x86_zap_private_spte)(kvm, gfn, level);
> > +       /* Because write lock is held, operation should success. */
> > +       if (KVM_BUG_ON(ret, kvm))
> > +               return;
> > +
> > +       ret = static_call(kvm_x86_remove_private_spte)(kvm, gfn, level,
> > old_pfn);
> 
> I don't see why these (zap_private_spte and remove_private_spte) can't be a
> single op. Was it to prepare for huge pages support or something? In the base
> series they are both only called once.

That is for large page support. The step to merge or split large page is
1. zap_private_spte()
2. tlb shoot down
3. merge/split_private_spte()
-- 
Isaku Yamahata <isaku.yamahata@...el.com>